High-purity ferromagnetic sputter targets and method of manufacture

ABSTRACT

The method manufactures high-purity ferromagnetic sputter targets by cryogenic working the sputter target blank at a temperature below at least −50° C. to impart at least about 5 percent strain into the sputter target blank to increase PTF uniformity of the target blank. The sputter target blank is a nonferrous metal selected from the group consisting of cobalt and nickel; and the nonferrous metal has a purity of at least about 99.99 weight percent. Finally, fabricating the sputter target blank forms a sputter target having an improved PTF uniformity arising from the cryogenic working.

FIELD OF THE INVENTION

This is a divisional of prior U.S. application Ser. No. 10/158,164,filed on May 31, 2002 now U.S. Pat. No. 6,652,668. In particular, thisinvention relates to low-permeability cobalt and nickel sputter targets.

BACKGROUND OF THE INVENTION

In recent years, manufacturers have relied upon several processingtechniques to manufacture sputter targets from pure cobalt and purenickel. Manufacturers have traditionally relied upon a combination ofhot working and cold working to lower sputter targets' permeability andincrease its magnetic pass through flux (PTF). Unfortunately, theseprocesses have limited success with respect to controlling thehigh-purity target's final magnetic properties. The target's highmagnetic permeability and low PTF in turn limit the target's usefulthickness to a relatively thin cross section. Furthermore, because theperformance of a ferromagnetic sputter target is extremely sensitive tominor variations in magnetic properties, production of acritically-uniform ferromagnetic target is also challenging. Finally,the magnetic properties of a ferromagnetic sputter target are themselvesa means to an end—the ultimate measure of improvement is the performanceof the target in a sputtering system.

Kano et al., in EP 799905, recognized that strain can manipulate ahigh-purity cobalt target's permeability. This patent publicationdiscloses a process that relies upon either cold or warm rolling toreduce the target's initial permeability parallel to the target'ssurface to about 7. Unfortunately, this process also increases thepermeability perpendicular to the target's surface.

Snowman et al., in U.S. Pat. No. 6,176,944, disclose another process forreducing permeability of high-purity cobalt targets. This process reliesupon: i) controlled cooling to produce an hcp structure; ii) hotworking; iii) further controlled cooling to reproduce the hap structure;and iv) cold working to lower the target's permeability. This processlowers the target's initial permeability to less than 9. The cobalttargets produced by this process, however, do not suffer from the severeanisotropic magnetic permeability of the Kano et al. process.

Lo et al., in U.S. Pat. No. 5,766,380, entitled “Method for FabricatingRandomly Oriented Aluminum Alloy Sputtering Targets with Fine Grains andFine Precipitates” disclose a cryogenic method for fabricating aluminumalloy sputter targets. This method uses cryogenic processing with afinal annealing step to recrystallize the grains into a desired texture.Similarly, Y. Liu, in U.S. Pat. No. 5,993,621, uses cryogenic workingand annealing to manipulate and enhance crystallographic texture oftitanium sputter targets.

Sawada et al., in Japan Pat. Pub. No. 3-115,562, disclose a cryogenicprocess for lowering the permeability of cobalt alloy targets. Thesecobalt alloy targets contained a combination of fcc and hcp phases. Thisprocess used cryogenic working at a temperature of −196° C. to furtherreduce magnetic permeability of the two phase cobalt alloy target.

Researchers have explored using cryogenic working to increase theforming limits of aluminum alloy sheet panels. For example, Selines etal. disclose a cryogenic process for deforming aluminum sheet in U.S.Pat. No. 4,159,217. This cryogenic process increases elongation andformability at −196° C. In addition, metal sheet forming industries haveexploited high strain-hardening rates to extend the forming limits ofsheet metal and improve sheet metal strain accommodation uniformity.

SUMMARY OF THE INVENTION

The method manufactures high-purity ferromagnetic sputter targets bycryogenic working the sputter target blank at a temperature below atleast −50° C. to impart at least about 5 percent strain into the sputtertarget blank to increase PTF uniformity of the ferromagnetic targetblank. The sputter target blank is a nonferrous metal selected from thegroup consisting of cobalt and nickel; and the nonferrous metal has apurity of at least about 99.99 weight percent. Finally, fabricating thesputter target blank forms a sputter target having an improved PTFuniformity arising from the cryogenic working.

The method forms a high-purity nonferrous sputter target. The nonferroussputter target has a sputter source selected from the group consistingof cobalt and nickel. The sputter source has a top surface forsputtering metal atoms onto a substrate, a side edge, a purity of atleast about 99.99 weight percent and a uniform PTF. The PTF uniformityis less than about 3 percent of the average PTF for the formula providedin the specification.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic drawing of a sputter target used to illustrateacceptable locations for measuring PTF; and

FIG. 2 shows a plot of the sheet resistance (Rs) uniformity (% 1 sigma)as a function of target life for cobalt sputter targets produced by coldworking and cryogenic working.

DETAILED DESCRIPTION

The process provides a method for manufacturing high-purity cobalt andhigh-purity nickel sputter targets with fine grain sizes, lowpermeability, high PTF (PTF as measured with reference to ASTM standardF1761-96), and enhanced material property uniformity. The lowpermeability and high PTF increase the sputter target's performance andallow for the manufacture of targets having increased thickness. Inaddition to this increased thickness, the process improves PTF and grainsize uniformity throughout the target's matrix. This in turn facilitatessputtering with improved deposited wafer film uniformity throughout thetarget's life.

The structure-sensitive material property uniformity of a given metalworkpiece is highly dependent upon the uniformity of imposedthermomechanical treatments that affect microstructural evolution. Fordeformation sequences, great advancements in deformation uniformity canbe achieved by increasing the strain-hardening rate of the metal beingworked. Increasing the strain-hardening rate of a metal causes imposedstrain to be more evenly accommodated throughout the workpiece, andincreases overall deformed-state uniformity. In ferromagnetic sputtertargets the uniformity of the material magnetic properties is absolutelycritical for the performance of the target in producing uniformlysputtered films. Furthermore, metalworking conditions that maximizestrain-hardening rate ensure the most uniformly distributed deformation.Applying deformation at cryogenic temperatures is particularly effectivefor increasing the strain-hardening rate of high-purity metals. Forhigh-purity metals, which traditionally can exhibit particularly lowstrain-hardening rates, cryogenic working significantly increasesstrain-hardening rates and ultimately improves uniformity of theaccommodated deformation. For alloyed metals however, cryogenicdeformation is less effective at increasing strain-hardening rates abovethe inherent (relatively high) strain-hardening rates of alloymaterials. For nonferrous-ferromagnetic materials, increasing thestrain-hardening rate leads to uniform deformation and uniform materialmagnetic properties.

The process operates effectively with high-purity nonferrous metalscobalt and nickel—these metals' ferromagnetic properties limit sputtertarget design and operation. The nonferrous sputter target has a purityof at least about 99.99 weight percent. For purposes of thisspecification, all concentrations are in weight percent. Advantageously,the nonferrous targets have a purity of at least about 99.995 weightpercent and most advantageously at least about 99.999 weight percent.

For the high-purity cobalt targets, the process most advantageouslyfirst involves heating a cobalt target blank to a temperature of atleast about 417° C. At temperatures above about 417° C., the cobaltmatrix transforms into its fcc phase structure for improved workability.Advantageously, the hot working occurs at a temperature of at leastabout 500° C. for improved workability. In addition, it is possible tohot work the blank at a temperature as high as about 1450° C. But thesehigh temperatures often lead to uncontrolled grain growth. Mostadvantageously, the hot working occurs at a temperature between about600 and 975° C. Hot working in this temperature range provides arelatively fine grain size, good workability and equiaxed grains.

After hot working, cooling the hot worked cobalt target blank below atemperature of about 417° C. transforms any fcc phase structure in thecobalt matrix into its hcp phase structure. After cooling to roomtemperature, the cobalt target blank has an initial magneticpermeability of about 12 and no detectable amount of fcc phasestructure. At room temperature, the entire matrix has transformed bythis grain refinement process as described above into a stable hcp phasestructure, with fine equiaxed grains having an average size of less than100 microns. For purposes of the specification, grain size representsgrains measured according to ASTM E112-96.

For nickel sputter targets, the process simply involves hot working,usually hot rolling, at temperatures above the dynamic recrystallizationtemperature. At these temperatures, the hot working readily formsdesirable equiaxed grains. Most advantageously, the hot working refinesgrains to an average size of less than 100 microns.

Cooling the sputter target blank to a temperature below at least −50° C.and cryogenic working the cobalt or nickel target blank at a temperaturebelow at least −50° C. imparts strain into the grain structure. Theprocess provides a benefit to sputter target blanks having any grainsize, such as those of 500 microns or less. Advantageously, the cooledsputter target blank starts with a refined-grain structure. For purposesof this specification, a refined-grain structure is a grain structurehaving a shape or size determined at least in part by at least oneprocessing step, such as that obtained from cold working, hot working,combinations of cold and hot working, recrystallization or phasetransformation. Most advantageously the cryogenically cooled sputtertarget blank has a grain size of less than 100 microns.

The cooling medium may be any combination of solid or liquid CO₂,nitrogen, argon, helium, or other supercooled gas. Advantageously, theprocess cools the blank to about −80° C. Most advantageously, theprocess cools the blank to at least about −196° C. or 77 K. The mostpractical temperature for most applications is 77 K (liquid nitrogen atatmospheric pressure).

The cryogenic working imparts at least about 5 percent strain, asmeasured with respect to the thickness of the nonferrous target blank.For purposes of this specification, the strain represents engineeringstrain or change in thickness divided by original thickness.Advantageously, maintaining strain to a level less than about 20 percentreduces cracking for cobalt target blanks. Most advantageously, thestrain is about 7 to 17 percent for cobalt target blanks. In fact,cryogenic strains at levels above about 10 percent provide littleimprovement in reducing the magnetic permeability of the cobalt targetblank.

For nickel target blanks, a strain of at least about 5 percent willimprove the sputter target's PTF uniformity. But strains of at leastabout 20 percent and less than about 90 percent are more effective fornickel targets. Most advantageously, nickel sputter target blanksundergo a strain of about 40 to 80 percent.

In addition to total strain, it is important to deform the blank at areasonable strain rate. Advantageously, the strain occurs at a rate ofat least 0.05 s⁻¹. Most advantageously, the strain occurs at a rate ofat least 0.5 s⁻¹.

For cobalt targets, cryogenically reducing the thickness of the blankreduces the initial magnetic permeability to between about 4 and 9 asmeasured parallel to the top surface of the cobalt target blank andmaintains the magnetic permeability at levels between about 9 and 14 asmeasured perpendicular to the top surface of the cobalt target blank.Most advantageously, the initial magnetic permeability reduces tobetween about 4 and 8 as measured parallel to the top surface of thecobalt target blank and maintains between about 9 and 12 as measuredperpendicular to the top surface of the cobalt target blank. For nickeltargets, the cryogenic working reduces the permeability to between 10and 20 as measured parallel to the top surface of the nickel targetblank or levels comparable to those achieved with room temperaturerolling of nickel blanks. Typical permeability levels are about 14 to 17as measured parallel to the top surface of the nickel target blank andbetween about 14 and 17 as measured perpendicular to the top surface ofthe nickel target blank.

For both cobalt and nickel sputter target blanks, the residual stressespresent following cryogenic rolling deformation contribute to theoverall PTF of the target blanks. Therefore, any straightening or plateleveling required for subsequent manufacturing steps must beaccomplished without relieving these desirable residual stresses. Forexample, usage of automatic plate levelers that impose plastic bendingstrains should be avoided. Precise hand leveling, minimizing imposedplastic strains, are preferred. Although not easily controlled, it ispossible to straighten the blank at cryogenic temperatures withautomatic plate levelers.

The uniformity of magnetic properties can be expressed by comparing thePTF values at several locations at regular intervals on the target blanksurface. The uniformity of PTF can be quantitatively expressed by thestandard deviation of those measurements using the equation:

$\sqrt{\frac{{n{\sum x^{2}}} - \left( {\sum x} \right)^{2}}{n\left( {n - 1} \right)}}$where n is the number of measurements in different locations, n is atleast eight, and x is the PTF measurement. Referring to FIG. 1, asputter target 10's PTF measurements occur on top surface 14 of sputtersource 12 at each location at least 5 cm inward from the side edge 16 ofthe sputter source 12 before attachment to backing plate 18.Advantageously, the PTF uniformity is less than about 3.5 percent of thetarget's average PTF. Most advantageously, the PTF uniformity is lessthan about 3 percent of the target's average PTF. Further decreases inthe PTF uniformity as a percentage of the target's average PTF representadditional increases or improvement in the target's PTF uniformity. PTFmeasurements are done per ASTM standard F1761-96 with a 740 Gaussmagnet. Measurements cited here are for targets having a thickness of atleast 0.5 cm.

EXAMPLE 1

A cast section of high-purity cobalt, with a purity of 99.99 weightpercent, was first hot rolled upon reheating at a temperature of 800° C.The hot rolled cobalt workpiece was allowed to return to ambienttemperature via air-cooling. The refined microstructure target blankhaving a grain size of about 53 microns was then subjected to cryogenicrolling reduction.

The cryogenic working involved first immersing a target blank in liquidnitrogen until the liquid nitrogen surrounding its surface no longerboiled. Immediately after immersing room temperature metals in liquidnitrogen, the liquid adjacent to the metal underwent “film boiling”.During film boiling, the gas barrier limited heat transfer. As thetemperature of the workpiece decreased and the metal approached −196°C., the gas film barrier began to break down and the liquid contactedthe metal surface before boiling. Heat transfer was relatively rapidduring this “nucleate boiling” phenomenon. When the workpiecesapproached −196° C., an audible change in boiling state signaled thetransition from film to nucleate boiling. The cryogenic rollingreductions were taken in steps of no more than 0.13 mm per rolling pass,to a final total reduction of about 10% in the thickness direction. Thesputtering target blank that was processed with the abovethermomechanical sequence was bonded to a standard copper backing plateand machined into an Endura-style (round disk-shaped) sputtering target.A comparison of the present example processing and relevant materialproperties and the comparative example processing and properties ispresented in Table 1 below.

Initial permeability was measured using a vibrating sample magnetometer(VSM). PTF was measured using a Hall probe and gaussmeter on a 5.1 mmthick blank at 51 mm from the blank edge using a 740 Gauss magnet ineight different locations.

TABLE 1 Comparison of processing steps and cobalt target properties fora comparative process and the present invention. Value FabricationParameter/ Comparative Cryogenic Co Target Property Co Target TargetSlice Diameter  135 mm  135 mm Slice Thickness   38 mm   38 mm HotRolling Temperature 1150° C.  800° C. Reduction during Hot  85%  85%Rolling Reduction per Pass   1 mm   1 mm Cold Rolling Temperature  25°C. −196° C. Reduction during Cold  10%  10% Rolling Reduction per Pass0.13 mm 0.13 mm Initial Permeability  8.5  7.4 (Parallel) InitialPermeability 13.3 12.2 (Perpendicular) Average PTF  291 Gauss  350 GaussPTF Standard Deviation   12 Gauss   9 Gauss PTF Standard Deviation/ 4.1%2.6% Average PTF Average Grain Size  132 microns   53 microns AverageBlank Thickness  5.1 mm  5.1 mm Blank Thickness Standard 0.10 mm 0.05 mmDev.

As illustrated above, the process provides significant advantages overthe comparative example. It provides a significant decrease inpermeability, increase in average PTF, decrease in PTF standarddeviation, and decrease in thickness variation.

The enhanced cobalt sputter target was then sputtered on an Endura®magnetron sputtering tool at a power of 800 Watts with 51 standard cubiccentimeters per minute of argon flow in the sputtering chamber. Thedeposition time was 90 seconds and the wafer temperature was 20° C.Table 2 summarizes the results of the deposition uniformity of a 4 mmthick Endura cobalt target at 2-100 kWh. This target was 1 mm thickerthan a normal target. Referring to FIG. 2, the cryogenic working shows aclear advantage in target performance over the comparative example, asmeasured by lower Rs deviations and a longer target use life. Thisresulted in a significant decrease in thickness variation of thedeposited film even though the target was 1 mm thicker. For targets madeby the process of the comparative example, an additional 1 mm thicknesswould be accompanied by wider variation in deposited film thickness orthe inability to strike a plasma.

TABLE 2 Sheet resistance uniformity deviations (%1σ) at various spacingsfor a 4 mm thick Endura cobalt target at 2-100 kWh. Co target usagelifetimes are shown in kWh. W/C 2 kWh 10 kWh 20 kWh Distance Wafer WaferWafer Wafer Wafer Wafer (mm) #1 #2 #1 #2 #1 #2 45 1.96% 1.98% 1.93%1.88% 1.99% 1.93% 47 1.17% 1.89% 1.09% 1.09% 1.22% 1.12% 49 0.73% 0.78%0.76% 0.78% 0.79% 0.84% 51 0.89% 0.87% 1.16% 1.16% 1.08% 1.06% 53 1.79%1.71% 1.63% 1.58% 55 2.40% 2.41% 2.35% 2.22% 57 3.01% 3.00% W/C 30 kWh40 kWh 50 kWh Distance Wafer Wafer Wafer Wafer Wafer Wafer (mm) #1 #2 #1#2 #1 #2 45 2.38% 2.35% 2.45% 2.36% 2.53% 2.53% 47 1.44% 1.40% 1.58%1.55% 1.71% 1.72% 49 0.97% 0.93% 0.95% 1.05% 1.10% 1.08% 51 0.98% 1.09%0.76% 0.78% 0.77% 0.80% 53 1.56% 1.57% 1.34% 1.31% 1.30% 1.33% 55 2.20%2.21% 1.97% 1.99% 1.86% 1.72% W/C 60 kWh 70 kWh 80 kWh Distance WaferWafer Wafer Wafer Wafer Wafer (mm) #1 #2 #1 #2 #1 #2 45 47 2.35% 2.34%49 1.63% 1.48% 1.90% 1.97% 2.07% 2.05% 51 0.88% 0.98% 1.02% 1.06% 1.22%1.25% 53 0.70% 0.66% 0.62% 0.58% 0.69% 0.65% 55 1.25% 1.17% 0.95% 0.81%0.82% 0.71% 57 1.64% 1.50% 1.42% 1.34% 59 2.07% 2.05% W/C 90 kWh 100 kWhDistance Wafer Wafer Wafer Wafer (mm) #1 #2 #1 #2 45 47 49 2.54% 2.54%2.91% 2.86% 51 1.57% 1.53% 2.01% 1.92% 53 0.85% 0.86% 1.28% 1.20% 550.61% 0.63% 0.93% 0.85% 57 1.07% 1.11% 1.05% 1.09% 59 1.70% 1.76% 1.60%1.61% Note: W/C Distance represents the wafer to cathode distance.

EXAMPLE 2

The PTF of a cobalt sputter target, having a purity of 99.99 weightpercent, (prepared with a process similar to that described inExample 1) was measured following a leveling treatment on an automaticleveler, which accomplishes leveling by alternate reversing plasticbending. The average PTF was measured to be 289 Gauss for a 5.1 mm thickblank, which is significantly lower than that of Example 1 (350 Gauss).The uniformity of the PTF as reported by standard deviation wasexcellent, at 6 Gauss. The present example shows that cryogenicdeformation of the cobalt blank ensures uniform magnetic properties,which remain uniform even after additional plastic deformation byautomatic leveling. The present example also shows that the residualstresses introduced during cryogenic working are important in achievingthe highest possible PTF values for cobalt targets.

EXAMPLE 3

Pure Ni target blanks, having a purity of 99.99 weight percent, wererolled to an approximate reduction of 47% in the thickness direction,with an initial thickness of 9.53 mm and a final thickness of 5.08 mm.One blank was rolled at room temperature, and the other was rolledfollowing cryogenic cooling in liquid nitrogen. Both target blanks hadan average grain size of 77 microns. The PTF of the two blanks iscompared in Table 3 below.

TABLE 3 PTF mean values and standard deviations for Ni sputter targets.Cryogenic Rolled Cold Rolled Property Ni Blank Ni Blank Average PTF 170Gauss 174 Gauss PTF Standard  5 Gauss  11 Gauss Deviation PTF StandardDeviation/ 2.9 % 6.3 % Average PTF Permeability 14.2 16.0 (Parallel)Permeability 14.3 16.3 (Perpendicular)

Once again, the advantage of cryogenic deformation for increasing theuniformity of magnetic properties is exemplified in this example. Inaddition, cryogenic rolling appears to provide about the same level ofPTF benefit as cold rolling.

In summary, the invention provides a method for decreasing the magneticpermeability of high-purity-nonferrous-ferromagnetic targets. Thisdecreased permeability allows for the production of sputter targetshaving an increased thickness for longer target life—the ability tosputter a thicker target and achieve better deposition uniformity allowsfewer target changes in a given year. In addition to this, the processalso improves the uniformity of both the magnetic properties and thesputter target's performance.

Although the invention has been described in detail with reference tocertain preferred embodiments, those skilled in the art will recognizethat there are other embodiments of the invention within the spirit andthe scope of the claims.

1. A high-purity nonferrous sputter target, the nonferrous sputtertarget having a sputter source selected from the group consisting ofcobalt, the sputter source having a top surface for sputtering metalatoms onto a substrate, a side edge, a purity of at least about 99.99weight percent, a single phase crystallographic structure, and a uniformPTF, the PTF uniformity being less than about 3.5 percent of the averagePTF for the formula:${{PTF}\mspace{14mu}{uniformity}} = \sqrt{\frac{{n{\sum x^{2}}} - \left( {\sum x} \right)^{2}}{n\left( {n - 1} \right)}}$where n is the number of measurements at different locations, n is atleast 8, and x is the PTF measurement at each location 5 cm from theside edge, wherein the nonferrous metal sputter target deposits a filmhaving a sheet resistance Rs uniformity of less than 1.0 percent.
 2. Thesputter target of claim 1 wherein the metal sputter source is cobalthaving a permeability between about 4 and 8 as measured parallel to thetop surface of the metal sputter source and a permeability between about9 and 12 as measured perpendicular to the top target surface of themetal sputter source.
 3. The sputter target of claim 1 wherein the metalsputter source is cobalt and the PTF uniformity is less than about 3percent of the average PTF.